Borehole tubewaves, as they relate to seismic and borehole geophysical applications, are hydraulically propagating acoustic waves which propagate up and down oil well boreholes. The borehole fluid used in such applications serves as the tubewave propagation medium. The borehole casing and geologic formation immediately surrounding the borehole can effect the impedance of the propagating medium; accordingly, the borehole conditions can affect tubewave propagation, although the vast majority of the tubewave energy is present in the borehole fluid.
Tubewaves may be so prolific in certain borehole environments such that they can significantly mask the seismic signals of interest in a particular application.
Tubewaves can be generated either in a source or detector borehole. A source borehole is a borehole in which a seismic source is used to create seismic signals in and/or around a specific geologic formation of interest. Source boreholes are used in inverse vertical-seismic-profiling (VSP) and borehole-to-borehole seismics. The actual mechanisms involved in the creation and propagation of tubewaves are not fully understood; however, it is known that tubewaves are generated in source boreholes. A detector borehole is a borehole in which seismic detectors are used to measure a vector or scalar component of seismic signals propagating in the geologic formation immediately surrounding the borehole. Detector boreholes are used in VSP as well as borehole-to-borehole seismics. Detector borehole tubewaves appear to be generated as a result of seismic signals which impact the borehole at high angles of incidence.
Upon creation, tubewaves appear to travel up and down the borehole with little or not attenuation. Tubewave propagation is theoretically equivalent to electromagnetic propagation in a transmission line. When a tubewave encounters the upper or lower end of the borehole, most of the tubewave energy is reflected, resulting in a borehole tubewave traveling in the opposite direction. Some of the tubewave energy is radiated into the surrounding geological formation as a secondary source. Similarly, when tubewaves encounter borehole anomalies such as casing shoes or significant variations in the cement bonding which surrounds the borehole, a portion of the tubewave continues to propagate in its original direction, a portion is reflected and propagates in the opposite direction, and a portion is radiated as a secondary source.
Suppression of borehole tubewaves in seismic applications significantly enhances the signal-to-noise ratios attainable in the borehole environments, thus reducing or eliminating their masking effect on seismic signals of interest. Conventional tubewave suppression techniques of the prior art have primarily involved plugging of the borehole. Although this method may reduce or eliminate the amplitude of the tubewave propagating past the plug, it fails to completely solve the problem. The plug presents an impedance mismatch which is clamped to the borehole; accordingly, tubewave energy is can be emitted as a secondary source. Further, a mechanical plug is difficult to install and remove, thus complicating operations.
Alternative methods of tubewave suppression involve the use of side branches to the main pipe; these side branches may be orifices or may be Helmholtz resonators. Referring now to FIG. 1A, the use of a Helmholtz resonator H as a side branch to a main pipe P is shown. The hydraulic disturbance, or tubewave, is propagating in the main pipe with an incident pressure amplitude of p.sub.i. The impedance contrast Z.sub.b presented by the Helmholtz resonator allows some of the hydraulic disturbance to propagate past the resonator with a pressure amplitude of p.sub.t. A portion of the signal will be reflected with pressure amplitude of p.sub.r. The theoretical performance of this resonator configuration has been defined in the literature; see Fundamental of Acoustics, L. E. Kinsler, A. R. Frey, John Wiley & Sons, Inc., New York, 1962, pp. 186-213. The transmission coefficient, a.sub.t, is defined as p.sub.t /p.sub.i. Referring now to FIG. 1B, a graph plotting the transmission coefficient versus relative frequency for the Helmholtz resonator configuration of FIG. 1A is shown. This resonator configuration can provide significant reductions in the amplitude of hydraulic disturbances traveling past the resonator side branch but only for a very narrow band of frequencies. Thus, the Helmholtz resonator branch of FIG. 1A forms a narrow band notch filter.
Referring now to FIG. 2A, the use of an orifice O as a side branch to a main pipe is shown. The orifice of FIG. 2A is ideally a vent into an infinite compliance. Hydraulic disturbances within the main pipe P are attenuated as they propagate past the vent. For an orifice branch into an infinite compliance, the transmission coefficients, a.sub.t, is 1/[1+(K/k)],
The value k is the wavelength constant, and is defined as 2.pi.f/c, where f=frequency and c=velocity of propagation.
For an orifice branch with a pipe length much smaller than the orifice radius, K is defined as .pi.a/3.4S, where a is the orifice radius and S is the cross-sectional area of the main pipe.
Referring now to FIG. 2B, a graph of the transmission coefficient for the orifice branch with respect to relative frequency is shown. The orifice branch acts as a high-pass filter with respect to the hydraulic signal passing the branch, and reduces the amplitude of low-frequency hydraulic signals. Such a configuration is not practically possible because an orifice branch in an actual borehole would not serve as a vent to an infinite compliance.
Referring now to FIG. 3A, a side branch employing a modified Helmholtz resonator H' is shown. The modified Helmholtz resonator H' includes a gas-filled region G and a liquid-filled region L. This configuration closely approximates that of the orifice branch configuration, since the modified Helmholtz resonator's gas filled-region G enables the resonator to act like an infinitely large compliance for low volumetric changes of the gas. At very low frequencies, the volumetric changes experienced by the gas/liquid filled region will be such that it no longer behaves as an infinitely large compliance and the probe will exhibit a higher transmission coefficient. A graph of the transmission coefficient versus relative frequency for both the ideal response and for the modified-Helmholtz response is shown in FIG. 3B.
Hydraulic desurgers or accumulators have been used in a variety of oil field applications, most notably as mud pump desurgers. In general, these desurgers are similar to the modified Helmholtz resonator of FIG. 3A and are composed of a large gas-filled bladder which is exposed to the hydraulic field. The desurger bladder is highly flexible and serves only to separate the gas from the fluid. Because the desurgers are gas-filled and their volume relatively large, very small pressure changes at the fluid-gas interface result in relatively large volumetric changes of the gas. Thus, the desurger maintains a relatively constant pressure at the fluid-gas interface. ICO Inc., Houston, Tex. is currently developing a borehole oil well desurger which works on much the same principal as conventional desurgers. The device is attached to production tubing and placed at the bottom of the borehole. The purpose of the device is to reduce the amplitude of pressure surges on production formations associated with production pump jacks. The design is composed of an outermost cylindrical region which is gas filled. The oil production products flow through the center of the desurger. A high strength specially designed cylindrical bladder separates the production flow from the gas.
This constant pressure effect of the desurger can be observed when desurgers are used on positive displacement mud pumps. With mud pumps, large flow fluctuations in the output of the mud pump will cause very large pressure fluctuations. A desurger can reduce these variations by more than 90 percent.
Although the application of desurger techniques to the attenuation of tubewaves appears to be straightforward, many practical engineering and construction problems exist. For example, if a standard desurger with a bladder precharged or pre-pressurized to near surface pressure is used in a borehole tubewave environment, the bladder volume will be too small at borehole hydrostatic pressures to properly absorb the flow variations attributed to tubewaves. If a specialized rigid bladder is used to allow precharging to the expected borehole hydrostatic pressure while at the surface, the rigidity of the bladder will ultimately restrict the compliance of the desurger and reduce its overall effectiveness. An active desurger which varies the precharge on a flexible bladder so that it is always approximately equal to the borehole hydrostatic pressure would be relatively complex and would likely exhaust gases into the borehole upon retrieval of the probe.
In view of the foregoing discussion, a desirable tubewave damping probe would suppress a borehole hydraulic disturbance as it passes the probe without totally blocking the borehole and/or clamping to the borehole. It would also be desirable for a tubewave damping probe to have a flexible gas-filled bladder which maintains a constant pressure for the suppression of borehole hydraulic disturbances. Further, it would be desirable for a tubewave damping probe to have means for preventing the over-expansion of a gas-filled bladder in seismic applications, such as upon retrieval of a probe with a precharged bladder. In addition, it would be desirable for a tubewave damping probe to employ multiple damping devices to achieve improved suppression and to employ selective restriction of hydraulic flow past the borehole device to improve the high frequency performance.